Sensing instruments are used for sensing various different process variables, such as level of a process fluid or material in a process vessel. Many such instruments are of the intrusive type in which a sensing apparatus is exposed to the process fluid for sensing level. One such type of intrusive sensor comprises a magnetostrictive sensor. A magnetostrictive sensor has a probe including a magnetostrictive wire maintained under tension in a tube. The probe extends into the process vessel. A magnetic float is movable proximate the probe and floats atop the fluid in the vessel. An electrical pulse is transmitted on the magnetostrictive wire. The electrical pulse interacts with the magnetic field of the float, which creates a torque on the wire to produce a torsional force on the wire, thus initiating a torsional wave that propagates along the wire at the speed of sound. Typically, a pick up sensor is positioned at one end of the wire to sense the torsional wave on the wire. The elapsed time is measured between the launch of the electrical pulse and the signal from the pick up sensor. The distance between the magnet and the pick-up sensor is calculated from the measured lapsed time multiplied by the speed of the torsional wave, representing level.
Known magnetostrictive measurement instruments use piezoelectric crystals as a pick up sensor. Such piezoelectric crystals are generally polarized in the longitudinal direction, referred to as the K33 coupling direction in the field of electro-ceramics. One form of position sensors uses two K33 polarized crystals mounted perpendicular to one another extending radially from the wire so that a tensile load from the torsional wire is sensed at an edge of the crystals. In this arrangement, the tensile or compressive load on each crystal due to the torsional displacement of the wire acts on only one edge of the crystal. This can result in a noisy and low amplitude signal. Such an arrangement is sensitive to the position of the crystals and requires fine adjustments and tuning, making it difficult to manufacture. Such a sensor may falsely detect spurious vibrations in a magnetostrictive wire.
In another form of position sensor, a number of wedge shaped piezoelectric crystals are arranged in a circular array and are bonded between a metallic inner ring and a metallic outer ring using an electrically conductive adhesive. The symmetry of such a crystal array can cancel out spurious noise effects due to random vibration. However, the high number of specialized crystals required to build such an assembly make it expensive. Moreover, the inner and outer masses and bonding steps add to the total cost.
The present invention is directed to improvements in piezoelectric position sensors.